The present invention relates to a shadow mask for a color picture tube for use in color television and computer color displays.
In color picture tubes for color television and color displays, a shadow mask formed from a metal sheet provided with a large number of small holes is used so that electron beams are applied to predetermined phosphors. Because accelerated electrons also collide against portions of the shadow mask other than the apertures, the shadow mask is heated and distorted by thermal expansion. This may cause the electron beams to be gradually displaced relative to the phosphor screen, resulting in color shift in the colored image.
Accordingly, nickel-iron alloys having a low coefficient of thermal expansion are used in place of low-carbon steel sheets conventionally used. Among them, Invar, which is a nickel-iron alloy containing 36% by weight of nickel, is widely used.
A shadow mask made of a nickel-iron alloy is manufactured as follows. For example, an ingot produced by melting alloy components prepared in a predetermined ratio is hot-rolled to a predetermined thickness and then subjected to cold rolling, annealing, etc., thereby producing a sheet. Then, the sheet is coated with a resist, exposed in a pattern for forming predetermined apertures, and subjected to etching to form apertures. In this way, a shadow mask original sheet is produced.
To attach the shadow mask original sheet to a color picture tube, the shadow mask original sheet needs to be formed by pressing in conformity to the inner configuration of the color picture tube. However, the shadow mask original sheet is inferior in press formability. Therefore, in order to improve the press formability, the shadow mask original sheet is annealed in a reducing atmosphere at a temperature of the order of from 700xc2x0 C. to 1,000xc2x0 C. Thereafter, the shadow mask original sheet is pressed and then heat-treated in an oxidizing atmosphere to form an oxide film on the surface in order to prevent rusting of the shadow mask and to reduce undesired reflection occurring when electron beams collide against the shadow mask. Thus, the desired shadow mask is obtained.
However, the shadow mask original sheet annealing process not only increases the number of process steps for production of a color picture tube but also involves problems such as undesirable sticking of a shadow mask original sheet to another and distortion due to nonuniformity of heat treatment, thus causing the image display quality of the color picture tube to be degraded.
Under these circumstances, JP 5065598A proposes a material for a shadow mask that makes it unnecessary to perform annealing before press forming by setting the proof stress and elongation of a shadow mask original sheet made of a nickel-iron alloy to values within predetermined ranges, respectively.
However, it is impossible to obtain a shadow mask having satisfactory characteristics simply by setting the proof stress and elongation to predetermined values.
An object of the present invention is to provide a shadow mask of superior characteristics that does not need the annealing process for improving the press formability after the aperture-forming process.